This invention relates to impact printers, and more particularly to a character slug for impact printers providing more uniform print quality.
Impact printers receive their name from the use of hammers or the like to impact a slug against an ink carrier and a record medium--usually twenty-pound bond paper--backed by a platen. The platen is the anvil for the hammer's blow. The ink carrier is conventionally a ribbon, i.e., an elongated web impregnated with ink. The ink is transferred to the paper record medium when the two are brought into intimate contact under the blow of the hammer. Ink is released from the ribbon is raised character areas on the slugs corresponding to the shape of a character. Broadly, a serial impact printer is one in which a line of print is inscribed one character at a time. Classic examples of serial printers are the familiar office typewriter, teletypewriter printers and low speed computer output printers. Other classic impact printers include calculating machines such as adding machines and business accounting machines which use mostly numerical characters.
Printing elements for serial printers shaped generally in the form of a wheel or the like have been known for some time. By way of example, see U.S. Pat. Nos. 2,236,663 (1941), 3,461,235, 3,498,439 and 3,651,916. Presently there is marketed a serial printer which has a print wheel having a plurality of slugs located at the ends of spokes or beams extending radially outward from a hub. The print wheel is rotated by a servo mechanism to position selected characters opposite a hammer and ribbon at a printing station. This type of printer has enjoyed commercial success as an electronic printer capable of high speed and versatile operation. The print wheel it employs is basically a single element structure in that the beams and slugs are an integrally molded thermoplastic structure. This print wheel delivers superior performance with very favorable economics, i,e., the integral wheel is relatively inexpensive to manufacture. Nonetheless, when subjective standards of print quality are encountered in certain application, the integral-structure print wheel heretofore available does not always give the desired print quality.
In automatic text editing typewriter applications the demands on a print wheel are greater than in an electronic printer. In the text editing or office typing environment, the demands for high print quality cause the print wheel to be subjected to about ten times greater force due to about five times greater hammer energy compared to a printer operating as a computer output terminal, for example. Text editing machines include a printer, a keyboard and an electronic controller having some form of memory or storage. A typist enters character information into the memory and/or creates a copy on the typewriter printer at from 0.5 to 2.0 characters per second (cps). The typed information is manipulated by the electronics to correct errors and arrange format, and an edited document is automatically typed by the printer under control of the electronics at speeds upward of 15 cps. Clearly, in this environment, the print wheel is asked to perform in manual and automatic modes which are distinct if for no other reason than on basis of speed. Of course, the user generally expects like print quality whether the machine is operated at a 2 or 20 cps rate.
A plastic, integral print wheel performs satisfactorily in both the low and high speed and energy modes mentioned above but not with the same print or image quality over the same life span. Loss in image quality is generally judged as the first fall off in image resolution detectible by the unaided eye. The improved print wheel of this invention, on the other hand, performs excellently over even a broader range of operating conditions than those mentioned above.
Print wheels when used in either an electronic printer or an automatic text editing typewriter must be accurately positioned such that the blow of the hammer drives the character against the record medium uniformly over the character area. This requires close dimensional tolerances from the position of the hub or center section of the print wheel to the hammer, all in relationship to the platen; especially for asymmetrical character configurations where the hammer strikes the slug away from the centroid of the character. With regard to asymmetrical characters, one part of the character strikes the record medium unevenly resulting in poor print quality. In some applications of the electronic printer, where print quality is not as critical, this degradation of print quality may be acceptable. However, in the automatic text editing typewriter better print quality is required. To achieve high quality printing in the automatic text editing typewriter, each character of the print wheel is subjected to the greater impact forces to ensure that the entire character area uniformly contacts the record medium.